Cryptanalysis Types of Attacks Ciphertext-Only Attack Attacker knows ciphertext of several messages encrypted with the same key and/or several keys Recover the plaintext of as many messages as possible or even better deduce the key ( or keys) G Iven EK(P1, C2=EK(P2)CEk(P Deduce: EitherP, P2,P k; or an algorithm to infer Pi+from C+=Ek(Pi+v Known-Plaintext attack Known ciphertext/plaintext pair of several messages Deduce the key or an algorithm to decrypt further messages Given: P, C- Ek(P, P2, C2=Ek(P2)Pi, Ci-Ek(P) Deduce: Either k, or an algorithm to infer Pi from 1=E(P1+)Cryptanalysis - Types of Attacks • Ciphertext-Only Attack – Attacker knows ciphertext of several messages encrypted with the same key and/or several keys – Recover the plaintext of as many messages as possible or even better deduce the key (or keys) – Given: C1 = Ek (P1 ), C2 = Ek (P2 ),...Ci = Ek (Pi ) Deduce: Either P1 , P2 ,...Pi ; k; or an algorithm to infer Pi+1 from Ci+1 = Ek (Pi+1) • Known-Plaintext Attack – Known ciphertext / plaintext pair of several messages – Deduce the key or an algorithm to decrypt further messages – Given: P1 , C1 = Ek (P1 ), P2 , C2 = Ek (P2 ),...Pi , Ci = Ek (Pi ) – Deduce: Either k, or an algorithm to infer Pi+1 from Ci+1 = Ek (Pi+1)